1. Field of the Invention
The present invention relates to bending presses such as press brakes for bending sheet-like workpieces such as sheet metals and, more particularly, to hydraulic bending presses in which the ram carrying a bending tool is vertically operated by hydraulic means.
2. Description of the Prior Art
As is well-known, a bending press such as a press brake for bending sheet-like workpieces such as sheet metals is provided with a pair of elongated upper and lower bending tools which are horizontally disposed in vertical alignment with each other. One of the upper and lower bending tools is horizontally fixed, while the other is so arranged as to be vertically moved by a ram toward and away from the fixed bending tool with its length kept horizontal to bend a workpiece. Usually, the lower bending tool is fixed and the upper bending tool is horizontally mounted on the ram to be vertically moved toward and away from the lower bending tool, but, in some bending presses, the upper bending tool is fixed and the lower bending tool is movably held by the ram. In either case, the workpiece to be bent is placed or held on the lower bending tool, whether the lower bending tool is fixed or movable, so that the workpiece may be bent when either of the upper and lower bending tools which is movable is vertically moved by the ram toward the other tool which is fixed. Generally, the ram is of a rigid plate-like member which is provided at one of its edges with a straight surface to hold the movable bending tool horizontally, and it is vertically disposed with its flat sides vertical in such a manner as to hold the movable bending tool in vertical alignment with the fixed bending tool. Also, in many bending presses, the ram is hydraulically operated by hydraulic means such as hydraulic cylinders and is guided by guide means such as guide bars and rollers to vertically move the movable bending tool toward and away from the fixed bending tool.
The bending presses of the above described construction are classified broadly into two types according to the manners in which the ram is pressed by the hydraulic means and is guided by the guide means to enable the elongated movable bending tool to bend the workpiece in cooperation with the fixed bending tool which is also elongated. One of the types is so constructed that the ram is pressed by the hydraulic means and guided by the guide means at the central portion thereof, and the other type is so constructed that the ram is pressed and guided at both sides thereof.
The bending press in which the ram is pressed and guided at the central portion thereof is superior in bending accuracy, manufacturing cost and constructional simplicity to the type of bending press in which the ram is pressed and guided at both sides. Also, since the elongated fixed and movable bending tools are inevitably bent or curved more or less according to bending pressure during bending operations, it is desirable that the ram be pressed by a single hydraulic means so that the movable bending tool may be curved into an arched or bow-like shape together with the fixed bending tool to perform an accurate bending operation.
In general, however, the bending press in which the ram is pressed and guided at the central portion is additionally provided, for the sake of construction, with a single or plural auxiliary hydraulic means in addition to the main hydraulic means. Usually, two additional auxiliary, hydraulic means are symmetrically provided at the sides of the main hydraulic means which is located at the central portion of the ram.
One of actual disadvantages with the above described arrangement is that the movable bending tool will be bent by the plural hydraulic means into a wavy shape during a bending operation while the fixed bending tool will be bent into an arched shape. Accordingly, the movable bending tool will press the workpiece in the wavy state onto the fixed bending tool which will be bent into the arched shape with a result that the bending accuracy is decreased. Also, since many notches and holes are formed on the ram to dispose the plural hydraulic means, the ram will be inevitably uneven so much in section modulus over the length thereof so as to cause stress concentration. Thus, the notches and holes and the resultant stress concentration will cause undesirable effects on the ram to decrease the rigidity of the ram and the bending accuracy. Furthermore, it has been difficult to provide the guide means for guiding the ram in a desirable manner because of the additional hydraulic means. Therefore, there has been a disadvantage that reaction will occur against the offset load only at the guide means for the main hydraulic means located at the central portion of the ram when a smaller workpiece is being bent by the movable and fixed bending tools apart from or off the center of the ram. Thus, the guide means will fail to cope with the reaction and will allow the ram to slant and cause the movable and fixed bending tools to be out of parallel with each other so that the bending accuracy will be greatly decreased.